A known overheat or fire alarm system comprises a sensor tube in fluid communication with a pressure sensor, also known as a pressure switch module. The sensor tube commonly comprises a metallic sensor tube containing a metal hydride core, typically titanium hydride, and an inert gas fill, such as helium. Such a system is shown in U.S. Pat. No. 3,122,728 (Lindberg).
Exposure of the sensor tube to a high temperature causes the metal hydride core to evolve hydrogen. The associated pressure rise in the sensor tube causes a normally open pressure switch in the pressure sensor to close. This generates a discrete alarm. The pressure sensor is also configured to generate an averaging overheat alarm due to the pressure rise associated with thermal expansion of the inert gas fill. The discrete and average alarm states may be detected as either a single alarm state using a single pressure switch or separately using at least two pressure switches.
An example of a single alarm pressure sensor for use with a pneumatic fire/overheat detector is described in U.S. Pat. No. 5,136,278A (Watson). This detector uses two deformable metal diaphragms to form a pneumatic pressure sensor switch. The diaphragms are typically formed from TZM alloy discs which have been subjected to a pressure forming operation to achieve the required pressure set point for the sensor. After pressure forming, the resulting diaphragm is of the order of 5-10 mm in diameter.
Historically, the pressure forming of diaphragms has been carried out prior to final assembly of the pneumatic pressure detector. An improvement to this manufacturing process is described in US2009/0236205 (Nalla). This document discloses a method of performing the pressure forming operation after final assembly of the switch module. Despite this improvement the manufacture of sensors with a consistently repeatable pressure set point is a relatively time consuming and potentially costly procedure.
A further shortcoming associated with known designs is the relatively large internal free volume of the pneumatic pressure switch. Gas within the free volume of the pneumatic pressure sensor will reduce the pressure rise associated with expansion of the inert gas or evolution of hydrogen within the sensor tube. This will have a detrimental effect on the heat detection capabilities of the system. In addition hydrogen gas evolved during a discrete alarm condition may enter the free volume of the pneumatic pressure switch. This hydrogen gas is then no longer in physical contact with the metal hydride core and cannot be reabsorbed upon cooling. This will have a detrimental effect of the ability of the detection system to successfully reset after a discrete alarm event. Both of these effects are more significant for short sensor tube lengths.
The present disclosure seeks to address at least some of these issues.